Acidizing of oil wells became a fact of life in the production of crude oil about a half century ago when it was discovered that a solution of hydrochloric acid injected into the formation would increase the extraction of crude oil from the formation. Almost as soon, it was discovered that the injection of this acid carried with it into the formation an iron scale from the tubing through which the acid was injected into the formation. The iron scale was dissolved from the pipe and added to iron-containing minerals from the formation. As the acid became spent through the reaction with materials in the formation and the iron itself, the pH of the spent acid began to rise and consequently, the solubility of the iron in the acidizing solution began to decrease. Precipitation of ferric hydroxide and other iron containing compounds cause serious damage to the flow channels in the formation and often would result in the formation producing less, not more, crude oil. Early calculations show that enormous amounts of iron would be carried into the formation and precipitated unless something was done.
One of the early attempts is described in an article "Secondary Deposition of Iron Compounds Following Acidizing Treatments", C. F. Smith, C. W. Crowe, and T. J. Nolan III, SPE-AIME, Dowell Div. of The Dow Chemical Co., Journal of Petroleum Technology, September 1969. This early article suggested adding sequestering agents to prevent the precipitation of iron. In a later article "Evaluation of Agents for Preventing Precipitation of Ferric Hydroxide from Spent Treating Acid", C. W. Crow, Dowell Div. of Dow Chemical U.S.A., Member SPE-AIME, SPE-12497, indicated that fifteen years later, in 1984, the treatment of choice was still to add sequestering agents or iron stabilizers to the acids in order to prevent the precipitation into the formation. It is notable that one of the co-authors of the 1969 article is the author of the 1984 article.
Later, in a 1988 article, "Iron Control Additives for Limestone and Sandstone Acidizing of Sweet and Sour Wells", B. E. Hall, Chevron Geoscience Co., and W. R. Dill, Halliburton Services, Member of SPE-AIME, SPE-17157, continues to discuss the prevention or iron precipitation through iron sequestration using ethylene diamine tetraacetic acid (EDTA), a well-known chelating agent for iron. A number of other strong chelating agents are discussed in the article.
A consistent theme runs through the discussion of acid treatment of subterranean oil bearing formations relating to the problems of iron precipitation as is discussed considerably in the 1983 article, "The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations", H. O. McLeod, Jr., L. B. Ledlow, and M. V. Till, Conoco Inc., Members SPE-AIME, SPE-11931. This article discussed in detail the planning and design of acid treatments of formations and demonstrates the mindset of those skilled in the art with respect to the iron precipitation problem. It also recognizes the contribution to the problem of removal of mill scale from the interior surfaces of oil field tubing through which the acid is pumped for acidizing. A 1993 article in World Oil, "Remove Tubular Mill Scale to Improve Completions", Dave Bills, Ramco Tubular Services Ltd., Aberdeen, focuses on the mill scale contribution to the problem and suggests a number of recommendations for avoiding the contribution from the mill scale. While many of the suggestions have salutory applicability, they are either impossible or impractical to follow.
Accordingly, until now, for a half century the problem has existed with respect to the precipitation of iron hydroxide from the spent acid used in acidizing a formation. Another ineffective attempt to solve the problem was to treat the interior of the tubing with an acid solution to remove the mill scale and bring it back to the surface with sequestering agents, but in many cases even this is not feasible since many wells for which acidizing is a necessity, are completed in a way which prevents such circulation. Another attempt has been to push clean tubing down the interior of the production tubing such that production tubing is not contacted with the acid during the acidizing process.
As seen above, many attempts have been made to solve the problem of iron precipitation, but even after half a century the problem remains unsolved and a source of frustration for those in the oil production business.
Accordingly, it is an object of our invention to provide an improved method for the acidizing of producing formations penetrated by a wellbore such that at least the presence of mill scale in an acidizing solution is effectively eliminated.